1. Field of the Invention
The present invention relates generally to time division multiplexed access (TDMA) wireless communication networks such as Global System for Mobile communication (GSM) networks. In particular, some aspects of the present invention relate to solutions for adapting to upgrades or downgrades in multi-slot data calls in TDMA wireless communication networks.
2. Discussion of the Related Art
Perhaps the most prevalent type of TDMA wireless communication networks in use today are GSM networks. A GSM network may operate on any one of several different radio bands (also known as the frequency of the network), for example, standard 900 MHz (used mostly in original European GSM networks), 1800 MHz (used mostly in Personal Communications Networks/Digital Communication Systems PCN/DCS in the United Kingdom) and 1900 MHz (used mostly in Personal Communications System PCS in North America). GSM technical standards are provided by the European Telecommunications Standards Institute (ETSI) and can be obtained directly from ETSI via the web site: www.ETSI.fr.
Many GSM networks support the use of multi-mode, multi-frequency or multi-technology mobile stations that are capable of performing many advanced services. GSM mobile stations may combine many advanced services offered by GSM technology with the latest in computers, displays and other technologies. For example, multi-frequency GSM phones may allow a single handset to operate (roam) on GSM systems that have different frequencies, for example standard GSM (900 MHz), DCS (1800 MHz) or US PCS (1900 MHz). Multi-mode GSM phones may allow the same handset to access different radio technologies including Digital European Cordless Telephones (DECT). Multi-technology GSM phones may allow the handset to access and display different information sources (e.g., Internet web browsing).
The major parts of a GSM network are illustrated in FIG. 1. Mobile stations communicate with nearby radio towers called base stations. A plurality of Base Stations (BS) convert the radio signals received from nearby mobile phones through corresponding antenna systems. A Base Station Controller (BSC) coordinates one or more base stations. The BSC typically comprises a control computer, data communication facilities, and multiplexing and demultiplexing equipment. (Only one BSC is shown in FIG. 1 for the sake of clarity. A typical GSM network includes multiples base station controllers.) The base station controllers pass telephone calls from the mobile phones to a switching system called a Mobile-Service Switching Center (MSC). The MSC connects the calls to other mobile stations or routes the call to the public switched telephone network (PSTN) or other type of networks such as the Internet.
The MSC is also connected to several databases that contain customer information to check authorization for services (e.g., voice, messaging, data and image services) and process call features. The Authentication center (AuC) is a database and processing center that is used to validate the identity of mobile phones. The Equipment Identity Register (EIR) is a data base that holds a list of unauthorized (such as stolen) mobile phones. The Group Call Register (GCR) is a network database that holds the attributes for the set-up and processing of voice group and broadcast calls. The Home Location Register (HLR) database holds the detailed subscriber service subscription information. (In GSM networks, the mobile phones contain an electronic card, known as a subscriber identity module (SIM), to identify the subscriber.) The Visitor Location Register (VLR) is a database that holds temporary information about active subscribers that are operating within the control of that particular MSC.
Controllers in the MSC coordinate base stations, switching functions and network inter-connections. Communication controllers combine or demultiplex the channels from high-speed communication links. The A-interface between the MSC and each BSC consists of a Time Division Multiplex (TDM) Pulse Coded Modulation (PCM) digital transmission system. The interworking function (IWF) in the MSC is used to process and adapt information between dissimilar types of network systems. It permits compatibility between different types of networks such as computer networks, messaging services and Internet web servers. In the interface role, the IWF may buffer, filter or convert different types of information.
A GSM network uses a combination of time division multiple access (TDMA) and frequency hopping. One network channel can be made to support multiple data calls by dividing it up into a number of time slots (TSLs) of pre-defined intervals. A call from a mobile phone is typically assigned to one or more time slots. The interval of the time slots is so short that none of the mobile users on the same radio channel notice that they are transmitting or receiving on a fraction of the channel. In this manner, the capacity of the network is significantly increased over standard analog cellular, which requires that an entire channel be assigned for transmission of a call.
However, the channel usage between the base station controller and the MSC may frequently change as the result of new calls, terminated calls, or downgrades or upgrades in calls, particularly multi-slot calls. Traffic channels of a multi-slot data call are delivered within one timeslot in the A-interface, up to 4 subchannels per data call. Initially, subchannels are reserved from the lowest bit of the timeslot (i.e., bit 0) up towards the highest bit of the timeslot (typically, bit 7).
Recent features introduced in GSM networks include downgrades or upgrades for non-transparent multi-slot data calls. In a downgrade, one or more subchannels are taken away from the multi-slot data call and given to another call, thereby reducing congestion in a base station cell. The multi-slot data call continues even though its data rate has been reduced. In an upgrade, more sub-channels are allocated to a data call. When a downgrade or upgrade occurs, any of the subchannels can be affected. While a message is delivered to the MSC/IWF which may indicate the number of dropped subchannels, the message does not indicate which particular subchannels were dropped. There is no possibility of signaling the dropped subchannel numbers from the BSC to the MSC. The dropped subchannel(s) can be any one of the originally used subchannels. Furthermore, if the dropped subchannels cannot be identified, the MSC/IWF may transmit data to subchannels which have been dropped, causing greatly reduced data call performance.
These features may create inefficiencies in channel usage since there is no automatic solution provided in the GSM specifications. Accordingly, there is a need for methods and network equipment which provide for the automatic detection of the dropping of one or more subchannels from a multislot data call and the dropped subchannels (or remaining subchannels), as well as the modification or adaptation of channel usage in a GSM network. However, adding such signaling would necessarily require changes in the GSM specifications, a great deal of testing and difficulties in implementation.
Accordingly, various embodiments of the present invention are directed to a mobile switching center in a GSM network and to methods of automatically detecting and modifying the use of subchannels in a GSM network. In one aspect of the invention, a mobile switching center switches telephone calls in a time division multiplex access wireless communications network architecture. A communications controller in the mobile switching center is adapted to control communications between the mobile switching center and at least one base station controller. An internetworking function in the mobile switching center is adapted to transfer information between the mobile switching center and another network other than the wireless communications network. A compact data services unit (CDSU) causes said mobile switching center to automatically detect, whenever a downgrade procedure occurs during a data call switched through said mobile switching center, the subchannels of the telephone call dropped in the downgrade procedure network.
The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example.